Obsidian is a volcanic glass that occurs in many color. The one from this listing is Black Obsidian, which is commonly used to eliminate bad energies in oneself or environment. Obsidian brings wisdom and it's a very good stone for learning. It is like a psychic vacuum cleaner. It is also used for grounding and it is an excellent scrying. This is a very protective stone in all aspects; physical, mentally and spiritualy. It measures aprox 75mm.